Conventional semi-hollow self-piercing rivets are known in the prior art, as described, for example, in DE 10 2006 028 537 B3 and in DE 10 2005 052 360 A1. The known semi-hollow self-piercing rivets have a rivet shaft with a central shaft bore, which extends for the most part over more than 80% of the total length of the semi-hollow self-piercing rivet in the direction of the rivet head. Such self-piercing rivet geometries are, due to their length and due to the spreading behavior of the classic semi-hollow self-piercing rivets alone not suitable for a manufacturing of thin sheet metal joints.
DE 196 48 231 A1 describes a method for influencing the spreading behavior of a semi-hollow self-piercing rivet driven into a component. In order to influence the spreading of the semi-hollow self-piercing rivet during the joining process, the central shaft bore is completely or partially filled with a substance which acts hydrostatically during the riveting process. These hydrostatically acting substances are materials whose state of aggregation is solid at normal room and transport temperatures, but can be quickly converted into a liquid or pasty state of aggregation during processing. Depending on the degree of filling of the central shaft bore of the rivet shaft, the beginning of the spreading process of the rivet shaft is variable. Admittedly, in the presence of a filling substance in the shaft cavity, a broadening process of the rivet shaft takes place earlier than in the case of a spreading which is initiated exclusively by the action of a die or additionally by a sharpened rivet foot geometry. However, an adjustment of the spreading behavior by filling the central shaft bore of the semi-hollow rivet is not reliable in order to adjust the semi-hollow self-piercing rivet to different thicknesses and materials of a thin sheet metal joint to be manufactured.
For producing a self-piercing rivet joint in aluminum sheets, EP 0 833 063 A1 describes the use of a semi-hollow rivet, which consists of light metal. In particular, the light metal semi-hollow rivet comprises a blunt cutting edge and a conical cavity in the rivet shaft.
In order to achieve an advantageous spreading of the semi-hollow rivet during the joining process, the total rivet length of the light metal semi-hollow rivet meets the ratio 5:3 to the maximum shaft cavity length. This ratio was precisely chosen because, after piercing the upper aluminum sheet, the rivet shaft of the semi-hollow rivet does not spread sufficiently when the shaft cavity length is lower. If, however, the shaft cavity length is larger, this affects the riveting of the upper aluminum sheet. Furthermore, the tapered shape of the shaft cavity causes especially the thinner areas of the hollow shaft to be spread widely outwards in the radial direction. In this way, the spread-out shaft area reaches an extent which corresponds to the diameter of the semi-hollow rivet. This radial spreading is also supported by the material of the semi-hollow rivet, which consists of light metal, such as aluminum.
Joints made of high-strength steels are, according to DE 10 2013 020 504 A1, produced with a self-piercing rivet, the shaft of which has an axial recess of small axial depth at the end opposite to the rivet head. Compared to the length of the self-piercing rivet, the axial depth of the shaft bore or the axial recess is less than 30% of the rivet length. In addition, the self-piercing rivet consists of a stable steel material, since the spreading of the rivet shaft is less important in the described riveting method. Due to the relatively small axial depth of the shaft bore it is rather achieved that the self-piercing rivet joint is formed by a compressing process of the self-piercing rivet, which is caused by the counter-pressure of the high-strength steel of the workpiece arrangement. Thus, the self-piercing rivet geometry and joint described here follows a completely different joining system than what is described, for example, in EP 0 833 063 A1.
Despite the variety of joining elements known in the prior art, it is an object of the present invention to provide a semi-hollow self-piercing rivet which is particularly suitable for the manufacturing of joint connections in thin sheet stacks, preferably of light metal. Another object of the present invention is to provide a manufacturing method for such a joint and such a semi-hollow rivet.